diff --git a/metropolis/proto/common/common.proto b/metropolis/proto/common/common.proto
new file mode 100644
index 0000000..def91e8
--- /dev/null
+++ b/metropolis/proto/common/common.proto
@@ -0,0 +1,24 @@
+// Copyright 2020 The Monogon Project Authors.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+package smalltown.core.proto.common;
+option go_package = "git.monogon.dev/source/nexantic.git/metropolis/proto/common";
+
+enum TrustBackend {
+    DUMMY = 0;
+    TPM = 1;
+}